Wireless networks performance and security

Posted by | October 27, 2010 | Wireless scanners | No Comments


Wireless local area networks (Wlan`s) are based on the 802.11 protocol and have become the standard for wireless connectivity. These networks are generally called wi-fi. The new IEEE physical standard (802.11n) made possible the support distribution of high-throughput applications like video streaming. Wi-fi networks however are prone to RF (radio-frequency) interference that can significantly affect the performance of wlan`s. In order to minimize WLAN downtime an IT administrator need both real-time and historical data from RF interference to diagnose the network`s performance.

Well-known interferers with WI-FI networks.

A wi-fi network uses the unlicensed 2.5 and 5 GHz frequency bands which they share with other devices. Except Bluetooth devices, no other devices have the capability to work together with the wi-fi networks thus when an interfering device transmits energy in the channel that a WLAN is using for communication, it significally affects the network.
Here are some devices that commonly interfere with wlan`s:
– wireless video cameras
– analog cordless phones
– DSSS digital cordless phones
– wireless game controllers
– conventional microwave ovens
– FHSS digital cordless phones
– inverter microwave controllers
– motion detectors
– bluetooth devices
– wireless baby monitors

RF interference degrades wi-fi network signal

The interference of RF devices on Wi-fi networks depends on the type of the interferer, the RF characteristics of the devices and the positioning of the interferer towards the wlan device. The tests revealed that interferences can degrade wlan signal from 10% to 100%, depending upon the location and the type of interferer. All the devices mentioned above were responsible for signal losses up to 100%, only the bluetooth device showed a minimal 4% signal loss.

Interferences threaten the wireless network security

As wlan`s support more and more applications like voice and video communications, security issues must be kept under control as interferences may come from an intentional, malicious interferer or from an unintentionate source like a cordless phone. It is vital that a wireless local area network to be permanently monitored so that RF interferences are discovered instantly and real-time alerts are send to network administrators.

Bandspeed testing yielded similar results. We tested the effect of interference on the throughput between a WLAN AP and
a WLAN client device in a typical office environment, placing the interferer in two different locations: NEAR and FAR. For
the test, we used an 802.11n WLAN AP and client, functioning in the 2.4GHz frequency band and set to use 20MHz wide
channels. In all cases, the WLAN AP and the WLAN client were approximately 8 meters apart and separated by 3 walls. In
Understanding the Effects of RF Interference on WLAN Performance and Security, Rev 1.1 p. 4 © Bandspeed, Inc, 2010
the NEAR location, the interferer was also approximately 8 meters away from the AP and separated by 3 walls. In the FAR
location, the interferer was approximately 38 meters away from the AP and separated by 10 walls. See Figure 1 for the
office diagram illustrating the relative positions within the office.
Figure 1: Office diagram with device positions for throughput testing.
For our testing, we used Iperf to measure the throughput of the WLAN. Iperf is a commonly-used network testing tool. We
measured the average downstream TCP throughput from the AP to the client over 20 seconds, using a 128K window size
(all other settings set to default). We measured the throughput on channels 1, 6 and 11 in the 2.4 GHz frequency band, with
and without a specific interferer active in the background. We performed this testing with the following interfering devices:
wireless video camera, inverter microwave oven, conventional microwave oven, Frequency-Hopping Spread Spectrum
phone (FHSS), Direct-Sequence Spread Spectrum phone (DSSS), analog cordless phone and a Bluetooth device. In each
case, a baseline throughput measurement was taken first, immediately following a measurement with the interferer active.
The results of the testing in the NEAR and FAR positions are shown in Tables 1 and 2 respectively.
THROUGHPUT (Mbps)
Channel 1 Channel 6 Channel 11
INTERFERING DEVICE Off On % lost Off On % lost Off On % lost
Wireless Video Camera 63 1.5 98% 78 0 100% 68 68 0%
Inverter microwave oven 63 20 68% 78 6 92% 66 0.3 100%
Conventional microwave oven 63 58 8% 78 30 62% 66 19 71%
FHSS phone 77 16 79% 65 10 85% 67 15 78%
DSSS phone 63 0 100% 78 12 85% 66 66 0%
Analog cordless phone 63 0 100% 78 78 0% 66 66 0%
Bluetooth device 63 63 0% 74 74 0% 68 65 4%
Table 1: Throughput Measurements with and without Active RF Interference in NEAR position
THROUGHPUT (Mbps)
Channel 1 Channel 6 Channel 11
INTERFERING DEVICE Off On % lost Off On % lost Off On % lost
Wireless Video Camera 64 64 0% 77 0 100% 67 67 0%
Inverter microwave oven 63 63 0% 77 50 35% 67 2 97%
Conventional microwave oven 64 64 0% 77 60 22% 67 20 70%
FHSS phone 74 24 68% 68 20 71% 74 32 57%
DSSS phone 63 0 100% 77 77 0% 67 67 0%
Analog cordless phone 63 0.7 99% 77 77 0% 67 67 0%
Bluetooth device 63 63 0% 77 77 0% 67 67 0%
Table 2: Throughput Measurements with and without Active RF Interference in FAR position
Understanding the Effects of RF Interference on WLAN Performance and Security, Rev 1.1 p. 5 © Bandspeed, Inc, 2010
The loss of WLAN throughput due to the active interference in both the NEAR and FAR positions is graphically shown in
Figure 2.
Data with Interferer in NEAR Position
CHANNEL 1 CHANNEL 6 CHANNEL 11
0 25 50 75 100
BlueTooth
Analog Phone
DSSS Phone
FHSS Phone
Microwave Oven
Inverter Microwave Oven
Wireless Video Camera
Loss in Throughput (%)
0 25 50 75 100
Loss in Throughput (%)
0 25 50 75 100
Loss in Throughput (%)
CHANNEL 1 CHANNEL 6 CHANNEL 11
0 25 50 75 100
Loss in Throughput (%)
0 25 50 75 100
Loss in Throughput (%)
0 25 50 75 100
Loss in Throughput (%)
BlueTooth
Analog Phone
DSSS Phone
FHSS Phone
Microwave Oven
Inverter Microwave Oven
Wireless Video Camera
Data with Interferer in FAR Position
Figure 2: Loss of WLAN Throughput due to Interference
Of the devices tested, only the Bluetooth device had minimal impact on WLAN throughput. All of the other devices
significantly degraded the WLAN throughput, with some up to 100% for specific WLAN channels.
As shown in Figure 2, no WLAN channel is immune to interference. For each channel tested, there was at least one
interfering device that degraded the WLAN throughput by 100%. Several interfering devices, such as the inverter
microwave oven and the FHSS phone, significantly degraded WLAN throughput on all 2.4GHz WLAN channels when in the
NEAR position. The wireless video camera, inverter microwave oven, DSSS phone and analog phone blocked WLAN
transmission completely on select channels even when in the FAR position.
Managing WLANs to meet minimum throughput performance levels requires monitoring of the RF environment for interferers
and enabling a network administrator to identify sources of interference and intelligently adjust channel configurations for the
WLANs to avoid the known interferers in the environment. Ideally, this capability should be integrated into the WLAN to
allow these adjustments to be done automatically and dynamically.

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